Darkness - Bil

The reln, for as unreasonable as the concept is, has a stigma against as being the "evil" one for many many peoples from expansive cultures to the individual. From being the bottom of the cycle --associations of all sorts from lower class to pits to undesirables-- to its element being darkness --of which many fears emerge-- to being the coldest aspect of the universe, Bil surely is seen in a negative light, pun intended, in many cultures. These are just feelings and the facts are that it is one of the six Nurlen, a fundamental building block of the universe without which nothing can be, a truly neutral thing.
Bil is the nureln closest to the aorlen Dax, body and space. Despite being so close to substance and thus observation, it is one of the most enigmatic nureln. Compared to the rotation of its opposite, Light - Jyk, the motion of bil, push, is still the least understood of all motions. But, unlike its form and element, the sensation that bil provides life is so common that it would be difficult for studies not to have found anything on sight.

Time

Time is a spiral and the point of that spiral that Bil is is the trough: it is the bottom of time and the the cusp of ascent and descent. Like all reln, the time aspect of bil is not well defined by scientists. The portion it represents has been understood for eons, but the way it affects the world has only recently been better understood that the bottom of time tends to try and freeze. When a timeline of a being reaches its trough, it comes to a halt, death is its common name. But at the same time, from that trough, that stillness comes an ascent into a new life. When applying this to the suzal, the trough in time is the point most dispersed within the suzal, a point which is less dense and able to culminate in a new personality, a new being. However, bil is not death, correlation does not equal causation. Spirits, personalities, may persist after a true death, the separation of the body and the soul. In this case, the being has not reached their trough as they have not dispersed their uniqueness into the suzal at death. This is not common.

Form

The state of bil is one of absolute silence, an unchanging state that nearly mirrors the reln of Dax. Unchanging is not exactly true, as absolute stillness is only found in the Aldr K-kep and its pure dax shell. But bil is near stillness, making things so cold that they are less and less able to move. This may relate to its motion, scientists believe, as the motion-full existence of the other reln would not be able to exist near a space that would have them still. Regardless of speculation, bil is also one of the coldest reln, as it is a near absence of movement. Winter, night, underground, etc are all cold due to a greater amount of bil; Kag and Nep do not push bil away nearly as much as other reln do, since the two are closer to dax than to suz.

Sensation

The ability to perceive color, or sight to put it simply, is the most well-documented aspect of bil. The experience of a concentration of bil in a sight organ, such as the eyes, perceives the color of all things around it; some life forms have evolved two or more of these organs to judge distance. Life has refined this to 1, not see absolutely everything and only observe things that are dense enough to matter to the life form; and 2, to reduce the range of observation to a limited space, typically in front of the being. Both refinements helped not overwhelm the being with too much sensory to track. This is not true for all life.

Transparency

An object having density and an object being viewable are not one and the same. The transparency of glass and air are expressed via different mechanics. Air is not viewable due to a low overall density. Glass is transparent due to refractions of color bending that do not express their color in the direction of the observer. This is also why transparent objects that do have an angle in which they are visible.

Did you ever wonder why you can see your reflection in your shadow if you stare long enough? That's because blocking the ambient jyk from reaching a place leaves a hole where the ambient shadow can conglomerate. And this shadow has its density between you and the floor, or whatever you're looking at, subtly different depending on the vector between a part of you and a part of the floor. So, subtly, on a flat terrain, there's a density displacement that matches you, thus creating the reflection. This is also how mirrors work, but with some chemistry added in to make the reflection clear.

— The Witch of Knowledge

Eyes

Now, how eyes work: everything dense enough exerts a pressure to push away the element of bil. This pressure is then translated by the reln and transferred to the observer through its sensation, creating sight. So, say there's a wall in front of someone, it is painted red. The density of the paint will be what the person sees as it is the thing most close and dense enough to the observer. That wall of paint will exert its pressure on the plane of bil in the eye in a specific spot, creating the viewing of the wall. While the person may see the paint of the wall, it is their intelligence and understanding of the world at large that informs them the paint is a cover. More rudimentary life forms may not be aware there is something under the paint as it is the closest dense thing to them.
It's also because bil is used in the sight organ that most creatures cannot see in darkness. While all other reln push bil away, darkness attracts the like. Begins with the ability to perceive in the dark tend to have a more advanced sight organ that either reduces the pull of the ambient bil around them or that can detect it to filter it out.

Fun fact: color blindness is the organ missing some receptors for specific reln that others in a species would normally perceive.

Motion

The motion of bil is to translate things "away". What "away" means is still contentious as scientists and philosophers are not sure if it's observed or some other aspect that defines what is "away". As far as scientists have used their tools to observe and philosophers have pondered, this "away" point seems to be dictated by the chemical makeup of a substance, the substance exuding "pressure" on things around it. Bil enacts the translation of a vector that is away on other things, creating translation of space from one point to another. This is one of the most important motions for life forms as it allows for safe and negotiable interaction compared to the separation of Kag and the slipping of Nep which, while also allowing for translation, do so in a way that would lead to a world very different than it is currently.

Did you know, the core of the planet is a terrible pit of bil, pushing everything away into space? if it weren't for the Sakzyg Oiov, we wouldn't have a planet to live on

— Grandmother Wayfarer to a group of children

Emotion

Fear comes in the form of the unknown, fairly well metaphorized in the absence of visual confirmation of threats in the dark. Fear of stillness, of things not changing, of things never getting better (or worse), of a life un-experienced. Fear of lack of connection, of other things never being able to get close, always pushed away. I don't think it's The Death that comes, but The Fear.

— Defza

Fear is a useful emotion, creating scenarios where a lifeform may continue living, avoiding danger and end. Danger, for many creatures, can be stimulated by any of the senses, positioning fright to avoid the sense telling them of threat. This usefulness means that fear is often an overwhelming emotion, blotting out others to have the being act as soon as they can. This translates into anxiety, a fear without reason: anxiety, fear for things outside the immediate temporary placement, either reminders of the past or readiness for the future, which is also beneficial in moderation. However, like acute fear, anxiety can be overwhelming and inhibit a being's ability to perform as they otherwise would in a given scenario.

Color

The aspect of reln that bil dictates is the color. This makes sense since the sense bil provides is to experience color. The color of bil itself is purple/violet/pink. Due to the eye organ not being able to experience true bil, many see this color as something not real. Those who have the adapted organ to perceive bil also typically have the same capability of seeing the color as anyone else, as bil interacting with itself could not possibly act in the same way as it with things non-self.
That said, bil still has color and gives the universe color. Color, as a definition, is the attribute of a substance that outputs a distinct pushing force of one thing on another, or the pressure of the densest reln in a given chemical. This depth of a substance also means that there are shades between one color and another in a torus (interesting as that is the shape of Light - Jyk). Purple bleeds into red and blue while green bleeds into yellow and blue and so on. However, there is a third dimension to the depth of color, a spiral. This spiral allows for further blending, such as a red-blue that is just red and blue without any bil or purple. Or, an orange and blue shade that is uniquely identifiable and separate from any other color combination. The depth of the spiral is theoretically infinite with each blending able to be followed further down the spiral for esoteric colors. Most beings can only experience a shallow depth of the color spiral.

Shifting

Opalescence, refraction, and other phenomena of color change are due to the position and direction of the reln in a chemical. For opalescence or iridescence, the chemical makeup allows for a range of colors to exist in one space that profess differently based on the angle at which they are observed. Refraction acts similarly where the presence of an object that has a density only at a certain angle allows for things to pass through it, typically smaller things, and they bounce around, depending on the structure, and come out changed from this.

Body/shape

The body part associated with Bil is the skin and hair as it is what most beings will see of one another and what is most susceptible to change in color. The attribute of bil, which is its shape, is an enclosed lattice. What this is, well... Imagine a circle that has "an infinite amount of faces" that all equally point outward. This shape would be the opposite. An infinite amount of edges that don't have any faces in a defined space. In "theory" an object purely of this shape cannot be touched. It is a substance with as much surface as possible without any tangibility.

Element

Darkness, also known as shadow, is the elemental aspect of bil. This is another elusive aspect that has not received an adequate definition. That hasn't stopped people from observing and finding use for it. Spend enough time underground or otherwise in places with a lot of shadow and one would notice that the density of darkness in an area seems to move around throughout the day. This makes sense as jyk & bil have the highest repulsion for one another. As the saykZuyg spins around the planet, the more dense part of the ring pushes all shadow toward the fractured part. This has been a great use to subterranean species so they have an adequate sense of time. This was later captured in technology, and the clock was invented!

As for what darkness does to a chemical makeup, it ensures the arrangement of things, pushing things into place that is a local minimum, aligning with its aspect of time. It makes stable compounds and the cold ensures that chemicals do not instantly react as soon as they encounter the presence of something else. It keeps things in a state where the energy is low enough that things can interact in a way that allows for life as it currently works.